Optical pickup lenses in CD, MD, DVD, and various other optical disc systems and image pickup lenses in video cameras and general cameras are generally produced in the following manner.
First, molten glass is dropped from a nozzle tip and a glass in the shape of a droplet is formed (droplet forming). As needed the glass is ground, polished and/or cleaned to prepare a preform glass. Alternatively, molten glass is cast by rapid solidification to form a glass ingot and the glass ingot is ground, polished and cleaned to prepare a preform glass. Subsequently, the preform glass is softened by heat application, and press molded by a precision machined mold to transfer the surface profile of the mold to the glass, thereby producing a lens. Such a molding method is commonly called a press molding method.
In employing the press molding method, to precisely press-mold lenses while reducing the degradation of the mold, a glass having a glass transition point Tg as low as possible (at least not higher than 650° C.) is required. Various glasses for this are proposed.
If devitrification occurs during preparation of a preform glass, this means that the fundamental performance of a resultant mold-pressed lens is lost. Therefore, it is important that the glass should have excellent resistance to devitrification. Furthermore, with recent increasing awareness of environmental issues, there is demand for an optical glass in which no harmful substances, such as lead, are used as its glass components. In recent years, it has been considered that for the purpose of cost reduction, optical lenses, such as optical pickup lenses for various optical disc systems and image pickup lenses, are reduced in lens thickness or number of lenses used. In order to achieve such reduction in lens thickness or reduction in number of lenses, a glass material is desired which has a high refractive index and high dispersion (i.e., a small Abbe's number). Examples of proposed glasses having these optical properties include optical glasses containing bismuth as a main component (see, for example, Patent Literatures 1 to 3).